This invention relates to an idling rotational speed control system for controlling the idling rotational speed of an internal combustion engine after cranking, and more particularly to a system of this kind which is intended to improve the stability of the engine rotational speed when the engine is at idle after cranking.
A system for controlling the idling rotational speed of an internal combustion engine has already been proposed by the assignee of the present invention in Japanese Provisional Patent Publication (Kokai) No. 62-3147, which controls a control valve for adjusting the opening area of an intake air passage bypassing a throttle valve after cranking of the engine, by the use of a feedback control gain which is set to different values depending upon whether or not a predetermined time period has elapsed after the cranking.
The proposed system is able to carry out more suitable engine rotational speed control than conventional ones. However, the proposed system still has room for improvement in the stability of the engine rotational speed when the engine is in the transitional state from cranking to after-cranking, as follows:
When the engine is brought into the aforesaid transitional state, the engine rotational speed abnormally increases, as shown by the broken line in FIG. 5, and consequently a drop occurs in the control amount for the control valve, as shown by the broken line in FIG. 6. Therefore, the proposed system still suffers from instability in the engine rotational speed at an early stage of the idling rotational speed feedback control after completion of cranking. Particularly in the case where the desired idling rotational speed is set at a higher value to promote the rise in the engine rotational speed immediately upon transition from cranking to after-cranking, the aforesaid abnormal increase in the engine rotational speed becomes even greater, which results in marked instability (fluctuation) in the engine rotational speed. Therefore, it has been difficult to secure stability of the engine rotational speed at the early stage of the idling rotational speed feedback control after completion of cranking.